BusinessHours bh = [SELECT id,SundayStartTime, MondayStartTime, TuesdayStartTime,
     WednesdayStartTime, ThursdayStartTime, FridayStartTime,
     SaturdayStartTime, SundayEndTime, MondayEndTime,TuesdayEndTime,
     WednesdayEndTime, ThursdayEndTime, FridayEndTime,SaturdayEndTime
   From BusinessHours
   Where IsDefault=true];

Datetime inTime = custom date time field 1
Datetime outTime = Custom date time field 2;

long businessMillisecondsDiff = BusinessHours.diff(bh.Id, inTime, outTime);
decimal businessHoursDiff = businessMillisecondsDiff / (1000.0*60.0*60.0);

custom_date_time_field__c = businessHoursDiff ;